The prior art provides digital feedback systems having a detecting means which outputs an increment signal or a decrement signal according to a predetermined amount of increase or decrease of a controlled variable, respectively, and having two kinds of feedback loops, along one of which feedback loops each increment signal is counted as +1 and each decrement signal is counted as -1, an aggregate of the count is compared with a target value of a controlled variable, a deviation from the target value is amplified in a low response speed amplifying means in which an amplifying operation at a low response speed and including an integration is carried out, and along the other of which feedback loops the count is directly amplified in the high response speed amplifying means in which an amplifying operation at a high response speed and not including an integration is carried out.
FIG. 1 shows a general construction of the conventional system as mentioned above.
In FIG. 1, a controlled variable detecting means 4 detects a predetermined amount of increase or decrease of a controlled variable, and outputs an increment signal or a decrement signal according to the predetermined amount of increase or decrease of a controlled variable, respectively; a simple counting means 5 counts each increment signal as +1 unit, and counts each decrement signal as -1 unit; a controlled variable target value command means 9 commands the target value of the controlled variable; a first deviation detecting means 6 detects deviation of the output of the simple counting means 5 from the target value; a high speed feedback amplifying means 2 amplifies the output of the simple counting means 5 by an amplifying operation at a high response speed and not including an integration; a low speed amplifying means 7 amplifies the output of the first deviation detecting means 6 by an amplifying operation at a low response speed and including an integration; a second deviation detecting means 8 detects the deviation of the output of the high speed feedback amplifying means 2 from the output of the low speed amplifying means 7; and a controlled variable varying means 3 increases or decreases the controlled variable according to the output of the second deviation detecting means 8.
In the construction of FIG.1, the output of the simple counting means 5 is amplified at a high speed in the high speed feedback amplifying means 2, and applied to the second deviation detecting means 8 as a feedback. If the deviation detected in the second deviation detecting means 8 is positive, the controlled variable varying means 3 operates to decrease the controlled variable, and thus the controlled variable is decreased. When the decrease of the controlled variable amounts to n units, the controlled variable detecting means 4 detects the change of -n units in the controlled variable, and the outputs n decrement signals. The simple counting means 5 counts the n decrement signals, and the output of the simple counting means 5 is amplified in the high speed feedback amplifying means 2 and applied to the second deviation detecting means 8. If the deviation detected in the second deviation detecting means 8 as a result of the change of -n units in the controlled variable is also negative, the controlled variable detecting means 4 operates to increase the controlled variable. Accordingly, the controlled variable is again increased, and the system detects the increase and twice attempts to decrease the controlled variable again.
As mentioned above, when the friction force operating against the change of the controlled variable is small, even if the system attempts to stop the controlled variable at a constant value, the controlled variable is repeatedly increased and decreased (vibrated) at the characteristic frequency in an unlimited manner.
In this case, only the loop which includes the high speed feedback amplifying means 2 has an influence on the vibration as described above, the signals from the loop which includes the low speed amplifying means 7, the operation of which includes an integration and the response of which is slow, has no influence on the above vibration.
For example, the phenomenon mentioned above occurs in the digital servo system of a motor, and small vibrations at the frequency of several tens to one hundred and several tens of Hz occur around the target value at which the controlled variable is commanded to stop.